The polymerization of alpha-olefins allows hydrocarbons to be obtained which are widely used as base for up-market engine lubricants, owing to their higher performance levels compared with mineral and semi-synthetic oils: higher viscosity index, better heat resistance, better oxidation resistance.
Numerous methods for synthesizing polyolefins have been described in the literature, using a varied range of catalysts, amongst which titanium-, vanadium-, zirconium-based catalysts associated with organic aluminum components.
Document EP 0 569 388 describes a formulation of catalysts intended to be used in a method to produce polyalpha-olefins of high molecular weight. Said catalyst formulation comprises a compound derived from a transition metal (Zr, Hf, Ti) and an aluminoxane. The method to produce a polyalpha-olefin described in this document consists of contacting one or more alpha-olefin monomers with a catalytically active association comprising said compound or said catalyst formulation, and collecting the poly alpha-olefin thus formed.
In the particular case of catalytic systems containing dimethyl diamide of titanium, interesting results reported in the publication by Scollard J. D. et al published in Macromolecules 1996, 22, 5241-5243 were obtained when polymerization of 1-hexene was conducted in the presence of a large excess of aluminoxane (tests n°1 and 2 in table 1 of said publication).
The use of aluminum activators in substantial quantity for the synthesis of polyalpha-olefins has numerous disadvantages however, leading in particular to pollution of the polymerization reactor, and brings constraints such as the subsequent removal of aluminum residues obtained after hydrolysis.
Also, the association of the dimethyl diamide complex of titanium with an activator of Lewis acid type such as tris-pentafluorophenylborane [B(C6F5)3] in the presence of a high quantity (500 equivalents) of methylaluminoxane (MAO) described in the document by Scollard et al gives unsatisfactory results in terms of molecular weight and activity (tests no. 5 and 6 in table 1 of said publication).
Complementary results published by the same authors in J. Am. Chem. Soc. 1996, 118, 10008-10009, describe the polymerization of alpha-olefins at room temperature in the presence of a dimethyl diamide complex of titanium and tris-pentafluorophenylborane as activator. However, the data given in table 1 of this document show that the molecular weight and the production activity of the poly alpha-olefins obtained remain limited.
The use of the dimethyl diamide complex however entails the prior synthesis of this compound which incidentally is highly sensitive. Additionally, the use of MAO in large excess makes the catalytic system more costly, and involves the subsequent elimination of aluminum residues obtained after hydrolysis.
Also, it appears that during polymerization reactions of alpha-olefins involving use of the dimethyl diamide complex of titanium, the results obtained are scarcely reproducible on account of the low stability of said complex.